...fake X post before date (7)
I believe the answer is:
pretend
'fake' is the definition.
(I know that fake is a more specific form of pretend)
'x post before date' is the wordplay.
'x' becomes 'ten' (X is the Roman numeral for ten).
'post' means one lot of letters go next to another (post- can mean after).
'before' becomes 'pre' (prefix meaning 'before').
'date' becomes 'd' (common abbreviation eg in DOB, date of birth).
'ten' put after 'pre' is 'preten'.
'preten'+'d'='PRETEND'
(Other definitions for pretend that I've seen before include "keep up appearances" , "Imagined" , "Imaginary, make-believe" , "Imagine" , "Sham" .)